Zinc is an indispensable component of all cells and is necessary for normal growth. Surprisingly little is known about the dietary requirements for zinc during infancy. This is partly due to wide variation in bioavailability of dietary zinc. Therefore, dietary requirements are likely to be different for breast-fed and formula-fed infants. Because so much of the required zinc is needed for growth, growth performance is a sensitive and important indicator of adequacy of the zinc supply in the infant's diet. Specific Aim 1 is designed to obtain more satisfactory estimates of zinc requirements of normal infants. This will be accomplished with the use of growth studies involving breast-fed and formula-fed infants. The specific hypotheses to be tested are (I) that breast fed infants are in marginal zinc status and will therefore show improved growth when given a small zinc supplement; and (II) that formula-fed infants will grow equally well when given only one-half the amount of zinc currently provided by formulas. Outcomes will be gains in length and weight, supplemented by indices of zinc status and quantitative measurement of zinc absorption with the aid of a stable isotope (70Zn). Although it is known that infants possess the ability to regulate the absorption and excretion of zinc, our knowledge in this area is very limited. Specific Aim 2 is therefore designed to define factors affecting (regulating) the absorption and excretion of zinc. We wish to explore in detail the adaptability of absorption and excretion of zinc in response to variation of zinc intake, and in response to dietary factors affecting the availability of zinc. These objectives will be accomplished in absorption studies using stable isotopes of zinc, and, in most cases, collection of feces at home with the use of diapers. Some of the objectives can be pursued simultaneously with objectives of Specific Aim 1. The results will provide a more complete understanding of the strategies used by the infant to acquire the quantities of zinc necessary for growth.